1. Field of the Invention
This invention relates to the field of electronic circuit design, and in particular to the design of a current mirror that provides a high output impedance and an accurate mirror of input current across a wide range of output voltages.
2. Description of Related Art
Current mirrors are often used to provide a controlled current to a component without loading the source of the controlled current. An independent source generates a current at a given value; the current mirror provides an output current to a load, such that the output current corresponds to the value of the independently generated current. In this manner, the source of the desired current is isolated from the load that receives an equivalent current.
FIG. 1 illustrates an example circuit diagram of a basic current mirror 100. A transistor T1 is configured as a diode, by connecting its drain and gate, for communicating the independent source current, Iin, to ground. A second transistor T2 has its gate connected to the gate of T1, and has its source connected to the same potential as the source of T1. Thus, the gate-to-source voltages of each of the transistors T1 and T2 are equal, and, if the transistors T1 and T2 are operationally identical, the drain-to-source current through each will be the same. The current through T1 corresponds to the input current Iin; therefore, assuming that the source of the current lout is sufficient to provide at least this value of current, the output current, lout, will be equal to Iin. Note, however, that the characteristics of the load that is intended to draw the current lout can affect the operation of transistor T2, by affecting transistor T2's drain-to-source voltage, Vout. If the drain-to-source voltage Vout of transistor T2 does not equal the drain-to-source voltage Va of transistor T1, the current lout through transistor T2 will differ from the current Iin through transistor T1. If Vout is less than Va, then lout will be less than Iin. Similarly, if Vout is greater than Va, then lout will be greater than Iin. This is due to the limited output impedance of transistor T2.
Output voltage compliance is defined herein as the range of output voltages through which a current mirror will provide an output current lout that corresponds to the input current Iin. The current mirror 100 exhibits relatively poor output voltage compliance, because only when Vout is equal to Va will the output current lout equal the input current Iin, due in part to the limited output impedance of the transistor T2.
FIG. 2 illustrates an example circuit diagram of a current mirror 200 that provides greater output impedance, and thus a wider range of output voltage compliance than the current mirror 100 of FIG. 1. In the current mirror 200, a differential amplifier Al and transistor T3 are configured to assure that the drain to source voltages Va, Vb of the input T1 and output T2 transistors are equal. The amplifier A1 and transistor T3 control the drain-to-source impedance of transistor T3, such that a controlled output current lout (=Iin) is provided independent of the output voltage Vout, when Vout is greater than Vb. Because the gate-to-source voltage and the drain-to-source voltage of each of the transistors T1 and T2 are assured to be equal, the output current lout is assured to be equal to the input current Iin, when the voltage Vout is greater than Vb. In the current mirror 200, the output impedance and voltage compliance is improved, compared to the current mirror 100, because in current mirror 200, the output current lout will equal the input current Iin whenever Vout is greater than Vb, which is set equal to Va. In this case, the voltage compliance is limited to the lower value of Va, which is generally determined by the source of the input current Iin.
FIG. 3 illustrates an example circuit diagram of a current mirror 300 that is operable to lower ranges of output voltages than the current mirror 200, as taught by U.S. Pat. No. 5,612,614, issued 18 Mar. 1997 to Barrett et al., and included by reference herein. In current mirror 300, transistors T1 and T4 are configured having a common channel and two gates, thereby forming a composite transistor. This composite transistor T1–T4 is diode-connected, by coupling the gates of each transistor T1, T4, to the drain of T4, thereby forming a two-input diode device that has an intermediate node between the gates that provides the drain voltage Va of transistor T1. By dividing the input source voltage Vc between the transistors T1 and T4, the voltage Va at the drain of transistor T1 is lower than the input source voltage Vc. The relative sizes/transconductances of transistors T1 and T4 determine the value of Va relative to Vc. Because the diode arrangement requires that the transconductance of transistor T4 be substantially higher than the transconductance of transistor T1, the value of Va relative to Vc is limited.